There are known several pretreatment methods for extracting crude oil from rice bran, such as for example a heat treating method involving heating and drying of the material rice bran at a temperature above 80.degree. C. or a cooking method where heating and drying are performed after adding steam or hot water to the material.
According to these known methods, the heat-instable components in the rice bran undergo the thermal changes (such as fixation of the coloring matter, promoted migration of the acetone insolubles into oil, etc.) under the actions of heat and oxygen in the air during the cooking treatment, resulting in production of only dark-brownish crude oil containing many impurities. Further, refining of such crude oil requires the completed processing steps, a large quantity of chemicals and much energy and labor and also involves the problem of generation of hard-to-treat soil water and a great volume of waste clay. Also, in the case of steam cooking, a peculiar offensive smell would be produced to cause environmental pollution.
In the extraction operation, it is common practice to pelletize the steamed rice bran with the object of promoting material migration in the liquid and hydro-extraction of the extracting solvent, but since a heat treatment with steam is involved in the process, it was impossible with such means to obtain crude oil with good quality. An ordinary type of screw extruder can be used for molding of said steamed rice bran because such steamed rice bran, unlike raw bran, is highly viscous and fluid and hence easily pelletizable. However, direct pelletization of raw bran by an extruder requires the treatment to be carried out under a high pressure because of poor viscosity and fluidity.
The dies employed in the commonly used screw extruders can not stand high pressures (above 400 kg/cm.sup.2), and if the die wall thickness is increased for providing endurance against such high pressures, the die slot is necessarily elongated to increase the surface area of the slot so that a higher pressure is required for accomplishing the desired extruction. Thus, great difficulties were involved with the use of a common type screw extruder for the pretreatment for extracting crude oil from rice bran.